Electrolytic cells generally consist of a layered structure having an anode and a cathode on opposite sides of an electrolyte. Such electrolytic cells can be used in oxygen concentrators and like devices to separate oxygen from an oxygen containing gas, such as air. Alternatively, electrolytic cells form the basis of solid oxide fuel cells. In both such devices, when the electrolyte is subjected to high temperature, it is capable of conducting oxygen ions. The electrolyte is formed of an ionic conducting ceramic material, typically, ytrium stabilized zirconia (“YSZ”) or gadolinium doped ceria (“CGO”).
In case of an oxygen generator, when an electrical potential is applied across the cathode and the anode and the electrolyte is subjected to high temperature, oxygen ions will be produced at the cathode. The oxygen ions will pass through the electrolyte and recombine at the anode to form molecular oxygen. In a fuel cell, fuel is passed on the anode side to react with the permeated oxygen from the electrolyte at operating temperature. An electric load can be placed across the cathode and anode for production of electrical power.
The electrolyte layer should be thin and defect free while the anode and cathode layers should be porous with minimum polarization resistance. The current procedure for fabricating an electrolytic cell include preparation of a dual-phase anode support layer, deposition of a dense film layer on the anode support to serve as the electrolyte and thereafter, the application of the cathode layer on the electrolyte layer. The porous anode support can be in the form of a disk or tube.
Various techniques have been used for fabrication of the dense, electrolyte film, including atmospheric plasma spraying, colloidal deposition, slurry/co-firing processes and tape isopressing. The slurry/co-firing process can be used to produce a dense electrolyte that has a thickness of about 25 microns. After the electrolyte is sintered, a porous cathode layer can be applied by slurry dipping.
There are two problems with the current manufacturing techniques used in the production of electrolytic cells. First, optimal performance is not produced because the cathode coating tends to have high polarization resistance. For instance, mixtures used in forming the cathode, such as lanthanum, strontium, cobalt, iron oxide (“LSCF”) and CGO, have poor pore size distribution and microstructure. This results in a lower triple phase boundary than is otherwise possible. Another problem is that electrolytic cells are prone to shorting after the cathode coating is applied. As a result of the shorting, the cathode layer has partial contacts with the anode layer which creates a short cut for the current path. When the electrolytic cell becomes shorted, it does not function. In fact, manufacturing yields for producing non-shorting electrolytic cells is low, only about 50 percent. The reason for the low productivity is believed to be related to the fact that the electrolyte film is typically thin, about 25 microns and cannot be sintered to 100% of the theoretically density used in currently available technologies. Thus, the electrolyte film contains a certain degree of defects that are of the sub-micron scale.
In typical cathode coating techniques, particularly those that involve application through a slurry solution, the fine particles that are contained within such solution are of the same scale as the defects existing within the electrolytic cell. Such fine particles are able to penetrate the defect and thus establish a short between the cathode and the anode.
As will be discussed, the present invention solves this problem by providing a method to make an electrolytic cell in which defects are filled in before cathode deposition to prevent the cell from shorting.